gsub not working throughout code snippet

Question

Using this code example

#!/usr/bin/ruby
paragraph = "champion xylophone excellent"
paragraph = paragraph.gsub("ch","tj")
words = paragraph.split(/ /)
words.each do |word|
   if word[0,1] == "x"
     word[0.1] = "z" 
   end
end
paragraph = words.join(" ")
paragraph.gsub("x","ks")
print paragraph

The output will be 'tjampion zylophone excellent' rather than 'tjampion zylophone ekscellent'

The same applies if the gsub is applied within the each to the individual words. I don't understand why it acts at the beginning but not at the end.

Edit

Second case is a distinct issue from the first:

#!/usr/bin/ruby
paragraph = "champion xylophone excellent"
paragraph = paragraph.gsub("ch","tj")
words = paragraph.split(/ /)
words.each do |word|
   if word[0,1] == "x"
     word[0.1] = "z" 
   end
   word = word.gsub("x","ks")
end
paragraph = words.join(" ")
print paragraph

Answer 1

When you first use gsub you are assigning it to paragrah

paragraph = paragraph.gsub("ch","tj")

The second time you are missing the assignment

change paragraph.gsub("x","ks") to

paragraph = paragraph.gsub("x","ks")